1. Field of the Invention
The present invention relates to a method for fabricating micro-lenses, and more particularly, to a method for fabricating micro-lenses by photolithography.
2. Description of Related Art
Presently, micro-lens array has been widely used in the field of image sensor design, such as CCD (charged coupling device) image sensor, and CMOS (complementary metal oxide semiconductor) sensor.
The objective of either the CCD image sensor or CMOS image sensor is to increase the incident light quantity per pixel so as to increase the sensing ability of the image sensor. Micro-lens array technique is frequently used in conjunction with the micro-lens array of the light sensing part per pixel, thereby raising the sensing ability of the high pixel sensing part.
In particular, the frequently-seen manufacturing method of the micro-lens array structure uses polymer thermal reflow to form all kinds of the required micro-lens shapes. However, as the pixel size shrinks or the pixel density increases, the gap between the adjacent micro-lenses will be too small and the temperature of the micro-lens substrate will be non-uniform, thereby causing problems such as incomplete lens thermal melting or improper thermal agglutinations between the adjacent micro-lenses in the conventional thermal reflow.
Referring to FIG. 1a, a first diagram of a conventional micro-lens thermal reflow process is illustrated. As shown in the diagram, when the micro-lens substrate is of non-uniform temperature, which is that the polymer material at a higher level of the substrate has been melted to form an ideal arc-shaped micro-lens 11 and the polymer material 12 at the lower temperature part of the substrate has not, an insufficient thermal reaction may occur, resulting in failure in forming an ideal arc-shaped lens of the polymer material.
Referring now to FIG. 1b, a second diagram of a conventional micro-lens thermal reflow is illustrated. As shown in the diagram, if heating continues following the process of FIG. 1a, the polymer material located at the part of a lower temperature of the substrate can also achieve a sufficient thermal reaction to form an ideal arc-shaped lens 13 by being melted. On the other hand, the micro-lenses located at the higher temperature of the substrate could possibly be thermally fused with adjacent micro-lenses 14.
In summary, it has become an urgent issue for designers in the micro-lens design field to propose a micro-lens manufacturing method to avoid incomplete thermal fusing or problems of agglutinating the adjacent micro-lenses in the conventional thermal reflow due to the undersized interspace of the adjacent micro-lenses and the non-uniform temperature of the micro-lens substrate after the pixel size is shrunk or the pixel density is increased.